Fine structures such as microstructures and nanostructures have applications in various fields such as photonic materials, micro-electromechanical systems (MEMS), biomaterials, self-assembly, etc. Recently, as an example of a technique for producing such fine structures, a continuous-flow lithography technique was proposed (D. Dendukuri, D. Pregibon, J. Collins, T. Hatton and P. Doyle, “Continuous-Flow Lithography for High-Throughput Microparticle Synthesis”, Nature Materials, vol. 5, pp. 365-369, 2006; US Patent Publication No. 2007-0105972, “Microstructure Synthesis by Flow Lithography and Polymerization). In the continuous-flow lithography technique, a photocurable fluid flowing in a microfluidic channel is exposed to a predetermined shape of light such that the photocurable liquid is selectively cured, thereby continuously producing a variety of free-floating microstructures.